Oligomeric flavanoids. Part 9. The first biflavanoids based on mopanol and peltogynol as inceptive electrophiles

Abstract

Continued investigation of the phenolic metabolities of Colophospermum mopane reveals the occur-rence of the first series of biflavanoids based on mopanol and peltogynol as electrophilic species. Amongst these are the promopanidins, (+)-mopanane-(4β,6)-fisetinidols (5) and (7), the epimopanane-(4,6 and 4,8)-fisetinidols (9) and (11), and the unique oxidatively coupled epimopanone-guibourtinidol (13). They are accompanied by the related propeltogynidins, (+)-peltogynane-(4β,6)-fisetinidols (15) and (17), epipeltogynane-(4α,6)-(–)-fisetinidol (19), and (–)-fisetinidol-(4α,6′)-(+)-peltogynol (21), the latter compound extending the rare series of biflavanoids with the equivalent of a ‘terminal’ flavan-3,4-diol moiety. The 2,3-cis-3,4-cis-(+)-mopanol (23) represents the first mopanoid/peltogynoid with (2R,3R) configuration. The structures of the (+)-mopanane-(4β,6)-fisetinidols are confirmed by synthesis, the forcing conditions of which are explained in terms of the conformational rigidity of the mopanol C-ring.